In the end you only need to run into the person who got out of bed the wrong side and you will have a problem.
And that's what drives me mad. Boarding a LHR flight out of DUB recently, I watched most other passengers boarding with two and more pieces of baggage. I was one of the last to board - boarding by row number means a more pleasant boarding experience (ha!) - and when I got on the aircraft I was told that my regulation size cabin baggage would have to go in the hold. I pointed out the two people ahead of me who happened to have three bags with them, and my bag stayed with me. Though the legs were a bit cramped by the time we got to LHR.
Point I am making is that it really is down to the individual at the gate. (And that goes for FR too).